Streptomycetes are Gram-positive, aerobic, filamentous soil bacteria, which belong to the order of actinomycetales. In an early stage of Streptomyces growth on a solid medium, spores germinate, and subsequently develop into a vegetative mycelium of multi-nucleoidal and branching hyphae with occasional septums (Chater and Losick, 1996). After environmental signals such as nutrient depletion, aseptate aerial hyphae are formed, growing on the vegetative hyphae, the latter being used as a substrate. Eventually, the aerial hyphae form uninucleoidal cells that develop into hydrophobic spores, which are budded off from the tips of the hyphae. One of the striking features of streptomycetes and other members of the order actinomycetales is their ability to produce a wide variety of secondary metabolites, including many antibiotics, which are produced in temporal relation to the onset of morphological differentiation in surface-grown cultures (Chater, 1989; Miyadoh, 1993). The molecular processes regulating the events that lead to differentiation of Streptomyces are presently only superficially understood, although new and interesting insights into the genetics of streptomycetes have come to light (reviewed in Champness and Chater, 1993; Chater, 1993).
Most streptomycetes only sporulate on solid media, while growth in liquid cultures is restricted to the formation of vegetative mycelium. This typically develops into intricate networks of hyphae, among others resulting in pellet formation, with only the most outwardly oriented sections showing high physiological activity, resulting in low yield of the desired product per unit of biomass. Furthermore, because of their filamentous morphology, high density fermentations of biotechnologically interesting streptomycetes often are highly viscous, resulting in a low biomass accumulation due to for instance aeration and mixing problems. From this perspective it is desirable that fragmentation of the mycelium in submerged cultures is stimulated, that branching of the mycelium is reduced and that in general the viscosity of the culture is reduced.
Cell division in all bacteria analysed so far involves the tubulin-like GTP-binding protein FtsZ, which polymerises into a ring at the prospected site of the septum, presumably forming the physical scaffold for the assembly of the cell division apparatus (reviewed in Lutkenhaus and Addinall, 1997). In Escherichia coli and Bacillus species many factors have been identified that are involved in cell division, but little is known about this process in actinomycetes. Here septum formation does not lead to actual cell division, and while in most bacteria ftsZ is essential, the gene has been shown to be dispensable for mycelial growth in Streptomyces coelicolor (McCormick et al., 1994).
In contrast to most actinomycetes, Streptomyces griseus shows the ability to sporulate in submerged cultures over a short time period, when grown in defined minimal media (Kendrick and Ensign, 1983; Ensign, 1988). Kawamoto and Ensign (1995a,b) identified a mutation in the gene ssgA that relieved repression of sporulation in rich media. SsgA encodes an acidic protein with a molecular mass of approximately 5 kDa that displays no significant homology to any other known protein in the database; in the sequenced genome of the actinomycetes Mycobacterium tuberculosis and Mycobacterium leprea no ssgA has been found (http//kiev.physchem.kth.se/mycdb). Overexpression of ssgA resulted in fragmented growth and suppression of sporulation in submerged cultures of S. griseus. Fragmented growth was also observed by Kawamoto and Ensign (1995b) by overexpression of ssgA in S.lividans, which was supposed to have an ssgA of its own on the basis of weak signals on a Southern blot. In S.griseus, Western blot analysis with polyclonal antibodies raised against SsgA revealed that expression of SsgA directly correlates to the onset of submerged sporulation, with the protein appearing shortly before spore formation (Kawamoto et al., 1997). Importantly, although sporulation and production of the antibiotic streptomycin are apparently linked in S. griseus, no suppression of streptomycin production was observed. Apparently, regulation of sporulation and antibiotic biosynthesis occur via separate pathways.